PMT pulse timing w a FADC

1
PMT pulse timing w/ a FADC

• FADC allows determination of pulse timing since
  several samples are taken during each PMT pulse
• Several algorithms are possible
• FEU-115 and FEU-84-3 are compared
• Sub-nanosecond resolution is possible

2
Full fit method

• A full fit of the pulse to the di-gaussian form
  is used
• Typical pulse and fit result is shown

3
Five-point method

• Pulse height is transformed to linearize via
• F(v)(-ln(V/Vmax)).5
• Five points are used to determine the time

4
Results for the FEU-84-3 (MC)

• Solid curve is full fit method, dashed is
  five-point method
• Full fit fails at low pulse height, defaults to
  bin center

5
Results for the FEU-115 (MC)

• Solid curve is full fit method, dashed is five
  point method
• Low pulse height remains difficult
• Performance generally inferior to FEU-84-3

6
Experimental results (84-3)

• A limited pulse library exists
• Sampled with 8 bit precision, 0.5 ns sampling
  period
• T0 is not known but is determined via fit to 9th
  deg. polynomial
• Peak, 75, 50 and 25 crossing are used

7
5-point peak finding

• The 5-point algorithm is used to locate the pulse
  peak time
• Fail only on rare events
• 426 ps resolution at 10 of full scale pulse.
  (better than MC)

8
5-point edge finding

• Compare 5-point time to time of 50 crossing
• Resolution slightly worse, 526 ps.
• 50 crossing jitters about 200 ps relative to the
  peak

9
2-point edge finding

• Use peak sample and sample 2 bins previous to
  interpolate to 50 crossing
• Always works, 293 ps resolution

10
3-point edge finding

• Use bins 1 and 2 samples before peak to
  interpolate to 50 crossing
• Always works, resolution is 160 ps at 10 of full
  scale
• A singe typical shower resolves event time to 65
  ps.

11
Other Features, 3-point method

• 25 crossing s159 ps.
• 50 crossing s157 ps.
• 75 crossing s279 ps.
• Peak time s339 ps.

12
Conclusion and Question

• 250 MHz, 8 bits seems to maintain all features
  of PMT pulses
• What feature carries the real timing
  information?